Add event type name validation & channel_type filter middleware

[seratch]

The Events API document says "you can subscribe message.channels events" but this could be confusing for developers. message.channels is not an event type but a concatenated string value {event.type}.{event.channel_type}.

I propose two solutions for improving developer experience in regard to this matter.

Add a simple validation for event type in constraints

Bolt framework can suggest the right way to configure message event constraints by raising a validation error.

Add a built-in middleware to filter message events with event.channel_type.

We may want to add a built-in reusable middleware to filter message type events by channel_type. But I think this is optional.

@aoberoi @stevengill @mwbrooks @misscoded
We may want to add the same validation to Bolt for JS and Bolt for Java. If there is no objection to it, I am happy to work on it. Thoughts?

Originally posted by @seratch in slackapi/bolt-python#201 (comment)

[stevengill]

This sounds like a great addition @seratch